In a known hydraulic system, a hydraulic accummulator is charged by a constant displacement pump to supply fluid to a load. The fluid flow of the pump is supplied to the accummulator. When the maximum accummulator pressure is reached, the fluid flow of the pump is returned via a bypass valve to the reservoir. While the pump operates through the bypass, the load is supplied with pressurized fluid from the accummulator. Accordingly the accummulator is discharged. When the accummulator pressure reaches a mimimum pressure allowed, the fluid flow of the pump is again fed to the accummulator. The valve can be either a special accummulator charge valve or a regular directional control valve which is controlled by pressure responsive switches. To avoid a permanent actuation of the valve, the pressure difference between the maximum and minimum pressure should be relatively large which results in a large variation of the accummulator pressure. Furthermore, the switching noise is remarkable.
According to a further known system the disadvantages above referred to can be avoided when the accummulator is connected to a variable displacement pump. Pumps of this type comprise a pressure control valve which controls the displacement volume of the pump such that the accummulator pressure is maintained constant. However, there is a disadvantage in that the pump must permanently operate against a high pressure which results in high hydraulic losses and excessive strain.